Hello!
I also just bought this Gothic Library and had trouble installing it. Here is what I did to finally get it in the game:
I had to install it from My documents/Electronic Arts/Sims 3/Downloads, opening the file GothicLibraryset.Sims3Pack and I found it in the launcher afterwards under the funny name "2". But as I used a saved game after this, I could not find it though I did have the other components of the package ready to buy. This time, I could not find it in my launcher while I was within the game.
I quit the game and started the whole thing again. Though I got "the package is already installed", I had the lot installed again in my downloads. Then I started the game again, started a new game, went to modify the town and cool, I had it in my commercial lots. I installed it, made a copy for the lots library, and when I opened my saved games I had it, with both the "2" copy and my saved copy in my commercial lots library.
Hope this will help.